Conventionally, a spindle motor as illustrated in FIG. 7 was typically used as an electric motor for rotating a 3.5-inch floppy disk in a floppy disk drive. In the illustrated spindle motor, a tubular radial bearing 3 made of oil-less metal is fixedly and directly secured, for instance by crimping, to a stator plate 1 made of ferromagnetic material and carrying an electric circuit thereon. A thrust ball bearing 5 is attached to the upper surface of the stator plate 1 so as to concentrically surround the radial bearing 3. The thrust ball bearing 5 comprises a retainer 5b rotatably carrying a plurality of steel balls 5a, and a pair of annular thrust plates 5c and 5d interposing the retainer assembly therebetween. The lower thrust plate 5c is secured to the stator plate 1 by a bonding agent or the like. A plurality of drive coils 7 are mounted on the upper surface of the stator plate 1 so as to concentrically surround the thrust ball bearing 5. A rotor shaft 11 fixedly attached to a rotor plate 9 is rotatably supported by the radial bearing 3, and the rotor plate 9 abuts the upper surface of the thrust plate 5d. An annular drive magnet 13 is secured to the rotor plate 9 so as to oppose the drive coils 7.
In this spindle motor, the rotor shaft 11 is supported by the radial bearing 3 with respect to the radial direction, and the rotor plate 9 is supported by the thrust ball bearing 5 with respect to the axial direction while the rotor plate 9 rotates around the pivot shaft 11. A floppy disk mounted on the back (upper) side of the rotor plate 9 by known retaining means not shown in the drawing can be rotatively driven as required.
However, according to this conventional spindle motor, because the radial bearing 3 supporting the rotor shaft 11 is directly attached to the stator plate 1 by crimping, the pressure applied to the stator plate 1 for crimping tends to produce deformation in the stator plate 1 so that the rotor shaft 11 may tilt or may be otherwise improperly installed, and this can seriously damage the commercial value of the motor. For instance, the rotor plate 9 may undergo a whirling movement, or the floppy disk may not turn in a prescribed plane.
Also, when the stator plate 1 made of ferromagnetic material is subjected to the magnetic attractive force of the drive magnet 13 attached to the rotor plate 9, the stator plate 1 may tilt around the thrust ball bearing 5, and a significant deflection may be produced in the part of the stator plate 1 adjacent to the radial bearing 3. This can lead to a significant tilting of the rotor shaft 11, and the floppy disk may not be turned in a prescribed plane as desired. In FIG. 8, the letter S indicates the displacement of the stator plate 1 that may be produced in a part near the radial bearing 3 in this manner.
Also, because the radial bearing 3 and the thrust ball bearing 5 are separately formed, and respectively attached to the stator plate 1, the dimensional and positional errors of these two parts may add up, and it is therefore highly difficult to ensure the perpendicularity of the rotor shaft 11 with respect to the thrust support surface (double dot chain line A in FIG. 9).